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Altitude effects on miniature diaphragm pumps

Edited by the Engineeringtalk editorial team Dec 27, 2005

It is important to consider how miniature diaphragm pump, compressor and vacuum pump performance is affected by the relationship between atmospheric pressure and altitude.

It is important to consider how miniature diaphragm pump, compressor and vacuum pump performance is affected by the relationship between atmospheric pressure and altitude.

Atmospheric pressure is caused by the weight of the air pressing down on Earth and on the subsequent air below.

Since the pressure depends on the amount of air above the point where the pressure is being measured, the pressure decreases as the altitude increases.

The air's pressure is related to its density, which is affected by its temperature, the amount of water vapour it contains and the height above the Earth's surface.

The lower the temperature, the slower the molecules that comprise air are moving which means they push less against their surroundings causing lower pressure.

A decrease in air pressure therefore has the effect of reducing the air's density and its respective mass for a set volume of flow.

Miniature diaphragm pumps, miniature air compressors and miniature vacuum pumps have a set compression stroke that produces a fixed volumetric flow regardless of altitude.

Since air is less dense at higher altitudes, the mass of the volumetric flow and therefore the ability to attain maximum sea level pressure or vacuum for a diaphragm pump is reduced.

For example, the same 26C saturated air at sea level takes 14.010ft3/lb dry air versus 16.953ft3/lb dry air at an altitude of 5000ft (1524m), or an increase of 21%.

In a similar manner, the density is reduced from 0.0765lb/ft3 (1.225kg/m3) to 0.0659lb/ft3 (1.056kg/m3), or 13.8% less, thereby reducing the air pressure drop for the same air flow volume.

This correlates to an approximately 13.8% reduction in pressure or vacuum performance from an air pump at an altitude of 5000ft (1524m) compared with if it was being operated at sea level.

Fluidic system designers need to be aware of the full range of altitudes their system may operate to ensure proper performance integrity at different locations.

A standard atmosphere table on the Hargraves website lists the percentage loss on a standard day for pressure and vacuum levels at varying altitudes.

Air pumps should therefore be sized with the necessary additional capacity at the maximum possible altitude.

Either a pressure relief valve or pressure regulator should be incorporated to bleed off the extra capacity at the lower altitudes.

Consider what would happen if a diaphragm pump, vacuum pump or micro air compressor is designed for a system without regard to possible high altitude locations where it might be operated.

For example, a miniature diaphragm pump is required to compress air to 20lb/in2(g) (1.38bar) and provide 10 litre/min flow for a product to function properly.

If this system was tested and verified at sea level and the diaphragm pump was not sized with additional capacity, then the product would not operate properly if operated at higher altitudes.

For instance, should this product be sold to operate in a large market such as Mexico City, it would be advisable to determine the respective pressure loss at this high altitude.

Using the altitudes of world cities table (also on the Hargraves website), it is noted that this city is located at an approximate altitude of 7400ft (2256m).

Referencing the standard atmosphere table, the effective pressure is determined to be approximately 24.5% less than the miniature diaphragm pump's specified pressure at sea level.

Instead of producing the necessary 20lb/in2(g) (1.38bar) of pressure, the diaphragm pump would only be able to produce approximately 15lb/in2(g) (1.03bar).

The miniature diaphragm pump should be sized at 26.5lb/in2(g) (1.83bar) to have enough capacity to compensate for the pressure loss at an altitude of 7400ft (2256m).